FIG. 1 shows a circuit arrangement having an inductive load, which is represented by an inductance L and an ohmic resistor R, and shows a current regulating arrangement for regulating a current IL through the load. This current regulating arrangement comprises a switch S which is connected in series with the load and which is used for connecting the load to a supply voltage V in pulsed fashion. With the switch closed, when approximately all of the supply voltage is present across the load, the current through the load rises in a sufficiently well known manner and falls when the switch S is opened, with the current through the load when the switch is opened being accepted by a freewheeling diode FWD which is connected in parallel with the load.
To determine the current drawn by the load, the load current is detected using a measuring resistor RS which is connected in series with the load and which is connected to an actuating circuit 2. This actuating circuit uses an actuation signal 21 to actuate the switch S on the basis of the voltage Vrs across the current measuring resistor RS, and hence on the basis of the load current IL, with the aim of regulating the mean value of the load current to a prescribed value.
To regulate the mean value of the load current IL to a nominal value I_nominal using a current regulating arrangement as shown in FIG. 1, two concepts are fundamentally known which are explained below with reference to FIGS. 2 and 3. The two figures show, by way of example, the respective time profile for the load current IL and for the actuation signal 21 for the respective method. For the illustration in FIGS. 2 and 3, it is assumed, in order to assist understanding, that the inductive load is an ideal inductance, which means that a triangular current profile is obtained when the switch S is turned on and off in pulsed fashion.
In the method illustrated with reference to FIG. 2, the switch S is respectively turned on when the load current IL has dropped to a lower limit value I_1, which is below the nominal value I_nominal by a firmly prescribed hysteresis value H, and turned off when the load current has risen to an upper limit value I_2, which is above the nominal value I_nominal by the hysteresis value H. In the case of this method, the hysteresis in the load current IL is constant after the upper limit value I_2 has been reached for the first time, and corresponds to twice the hysteresis value H.
The gradient with which the load current IL rises when the switch S is turned on is dependent on the supply voltage V to which the load is connected in pulsed fashion. In this case, the gradient of the load current IL increases as the supply voltage increases, which results in a shortening of the turned-on period and hence in an increase in the switching frequency, which is shown using the right-hand part of the time profiles in FIG. 2, in which the time profile of the load current IL and of the actuation signal is shown for a relatively high supply voltage V. By contrast, the gradient of the current IL when the switch is opened is essentially independent of the supply voltage.
In the case of the method explained with reference to FIG. 2, the switching frequency is very highly dependent on the characteristics of the load, of the switch S, of the freewheeling diode FWD, of the temperature and particularly of the supply voltage V. Since the switching losses in the switch S are dependent on the switching frequency, the switch S needs to be proportioned, for safety reasons, such that it is able to take on the high power losses which arise at high switching frequencies, which increases the manufacturing costs for the switch. Furthermore, the frequency of electromagnetic radiated interference resulting from the switching processes likewise fluctuates over a wide range in the case of this method, which makes it more difficult to suppress the propagation of this radiated interference.
In the case of the method illustrated with reference to FIG. 3, the switch is turned on at the rate of a firmly prescribed frequency. To set the turned-on period for the switch, the mean value of the load current IL is determined over one or more previous actuation periods and is compared with the nominal value I_nominal. In this case, the turned-on period is adapted on the basis of the determined comparison result, that is to say is increased in comparison with the previous value if the determined mean value is below the nominal value I_nominal and is reduced in comparison with the previous value if the determined mean value is above the nominal value I_nominal. The turned-on period can be regulated digitally or in analog fashion, for example using a proportional integral controller (PI controller).
A drawback of this method is that the transient response is worse than in the method with constant hysteresis. Furthermore, suitable proportioning of the control loop in order to attain sufficient stability is difficult and complex.